Nitride compound semiconductor materials are compound semiconductor materials which contain nitrogen, such as materials from the system InxAlyGA1−x−yN where 0≦x≦1, 0≦y≦1 and x+y≦1. In the present instance, the group of radiation-emitting and/or radiation-detecting semiconductor chips based on nitride compound semiconductor material include in particular semiconductor chips in which the epitaxially fabricated semiconductor layer, which generally has a layer sequence comprising different individual layers, contains at least one individual layer which has a material from the nitride compound semiconductor material system. The semiconductor layer may for example have a conventional pn junction, a double heterostructure, a single quantum well structure (SQW structure) or a multiple quantum well structure (MQW structure). Structures of this type are known to the person skilled in the art and are therefore not explained in any more detail at this juncture.
It is known for a semiconductor material to be grown epitaxially on a substrate whose lattice constant is matched to the lattice constant of the semiconductor material in order to obtain an improved crystal quality and fewer crystal defects. A lattice-matched substrate which is also sufficiently suitable for the mass production of semiconductor chips of this type has not been disclosed heretofore in the case of the nitride compound semiconductor materials. Therefore, substrates based on sapphire, silicon carbide or spinel are frequently used, even though their lattice constant is not optimally matched to that of nitride compound semiconductor material.
An additional problem is that the epitaxial growth takes place for example at a temperature in the range of approximately 800° C. to approximately 1000° C. and is subsequently cooled for further processing. In this case, different thermal expansion coefficients of substrate material and semiconductor material layer grown thereon may lead to thermally induced mechanical stresses, with the result that there is the risk of damage to the semiconductor material layer due to cracks, for example. Conventionally, it is attempted to reduce this risk by matching the thermal expansion coefficients of the semiconductor material layer and of the substrate material to one another as well as possible.
Since the number of different materials suitable for the substrate is comparatively small in the case of nitride compound semiconductor materials, the aforementioned thermal matching is possible only to a limited extent, however. This means that, on the one hand, the maximum layer thickness that can be achieved for the semiconductor layer is limited and, on the other hand, the yield of semiconductor layers grown in a manner free of cracks is reduced.
These problems of limited layer thickness and yields also relate to semiconductor materials grown on the conventional substrates sapphire, spinel or silicon carbide. While the thermal matching between the semiconductor layer and the substrate is still relatively successful in the case of the material sapphire, for which reason nitride compound semiconductors of sufficient layer thickness can be grown on sapphire, only very thin layers made of nitride compound semiconductor materials having a maximum layer thickness of 3 to 4 μm can be grown epitaxially in a manner largely free of cracks on a substrate made of silicon carbide. Since it is intended to use the nitride compound semiconductors to fabricate optoelectronic components, in particular semiconductor lasers, and since these components may evolve a high thermal power loss depending on the individual case, the material sapphire is of only extremely limited suitability for the fabrication of power laser diodes, on account of its poor thermal conductivity. The use of silicon carbide as a substrate material has the advantage of a good thermal conductivity.
It is furthermore known to use special deposition methods for reducing the defect density in the semiconductor material. A method of this type is often referred to as the LEO method (lateral epitaxial overgrowth) or the ELOG method (epitaxial lateral overgrowth) and is disclosed for example in Song et al., Phys. Stat. Sol. (a) 180, 247 (2000), the content of which in this respect is hereby incorporated by reference. The fabrication of a gallium nitride layer on a sapphire substrate is described therein. First of all a thin initial layer (seed layer) is applied on the sapphire substrate and a strip-type silicon nitride mask layer is applied to said initial layer. During the subsequent deposition of trimethyl gallium and ammonia, first of all a plurality of gallium nitride layers grow between the mask strips. As soon as the gallium nitride layers have reached the thickness of the mask layer, the epitaxy parameters are set such that lateral growth occurs in addition to the vertical growth. Consequently, the mask layer is laterally overgrown by the gallium nitride layers. This process is continued until a closed gallium nitride layer is produced.
It has been found that the dislocation density in the gallium nitride layer fabricated by lateral overgrowth is advantageously low and is distinguished by a higher crystal quality in particular compared with a layer which is grown directly on the sapphire substrate.
However, comparatively large layer thicknesses (approximately 3–10 μm) are produced during these methods with coalescing ELOG layers, with the result that, in particular when using SiC substrates, the risk of cracking is very high and, consequently, it is only with difficulty that optoelectronic components with a sufficient quality can be fabricated.